A fundamental factor limiting the performance of a digital communication system is the phenomenon known as inter-symbol interference, which is well known by a person skilled in the art. Inter-symbol interference causes at the receiver level temporal occupation of each symbol or bit transmitted, which is longer than the initial duration of the symbol. This is also referred to as the bit time, for example.
Stated otherwise, the signal received at a given instant does not depend on one symbol or bit alone, but also on the other symbols or bits sent, which extends over durations greater than those of one symbol or bit time. In practice, the signal received at a given instant depends on the symbol concerned, and also essentially on adjacent symbols.
The causes of inter-symbol interference are multifold. One of them is due in particular to the multiple propagations of the signal between the sender and the receiver when the signal is reflected or diffracted by various obstacles. This leads to the reception of several signal copies that are mutually shifted temporally. Moreover, this interference between symbols is produced not only by the propagation between the sender and the receiver, but also by the sending/receiving devices themselves, such as the modulator, filter, etc.
During communications with interference between symbols, the problem arises on estimating the impulse response of the transmission channel. The quality of this estimate depends on the capacity to eliminate the interference between symbols, and hence to take corrective action regarding symbols sent.
Generally, the estimate of the impulse response of the channel, or more simply the channel estimate, is performed within the GSM telephone domain by using least squares techniques, and by using a predetermined sequence of symbols which is known to the sender and to the receiver. This is referred to by the term training sequence, which is well known by a person skilled in the art. This training sequence is present within each symbol train or burst sent. When the characteristics of the channel are sufficiently well estimated, the estimated coefficients of the impulse response of the channel are used in an equalization processing operation, also well known by the person skilled in the art. This is done to decipher the signal received. That is to say, the logic values of the symbols or data sent in the train are retrieved.
The equalization processing operation is conventionally followed by the channel decoding processing operations intended for correcting any errors. Channel decoding is followed by another decoding, known as source decoding, which is intended for reconstructing the information (speech, for example) initially coded at the level of the sender.
The article by Khayrallah et al., entitled “Improved Channel Estimation With Side Information” (1997, IEEE) and the corresponding U.S. Pat. No. 5,838,739, discloses a method of channel estimation. This estimation is based on the combined use of training sequences, and of the known characteristics of the sending and receiving means (filters), and includes determining the estimated coefficients of the impulse response of the channel.
However, such a method has drawbacks. One of them resides in the fact that this method depends on the information transmitted, namely the training sequences, which may be different depending on the systems used. Consequently, it is necessary to store in the portable telephone, for example, as many matrices as there are different possible training sequences. Another drawback resides in the fact that such a method is not applicable to so-called blind channel estimates, that is to say, when training sequences are not available.